1. Field of the Invention
The present invention relates to an automatic screw cap tightening method for automatically tightening a screw cap to a container for medical supplies, cosmetics, foods or the like, and it relates also to an automatic screw cap tightening apparatus therefor.
2. Description of the Prior Art
In such containers to be provided with screw caps, it is necessary that the screw caps be tightened at a predetermined ideal torque. The reason for this is that, in the case where a tightening or screw torque applied to a container to be closed is smaller than necessary, there is a fear that a screw cap therefor would be loose during the preservation or transportation of the container. Also, in general, the closed container is opened by human hands. In the case where the tightening or screw torque applied to the closed container is larger than necessary, it would be impossible to manually open the container with ease. Also, if the container is tighten by the screw cap at a tightening torque larger than necessary, then the container to be tightened and the screw cap and/or packings or the like attached to the container and the screw cap would be damaged during the tightening operation. Also, the caps to be screw-fitted to the container are made of different materials such as glass, synthetic resin or the like and the tightening torque is varied in accordance with combination of the material. It is therefore necessary that the screw cap be tightened at a tightening torque suitable for material, configuration and size of the container to closed thereby.
More specifically, as shown in FIG. 1, the conventional tightening apparatus is so constructed that a container 1 to be closed is fixed on a container receiving base 2 by a drum portion retainer 3 which is so called a gripper; a closing cap 12 is held stationary by a fastening air chuck 7; the air chuck 7 is coupled to a spindle 8 through a frictional member 9 sandwiched by upper and lower retainer plates 10; and a rotation of the spindle 8 is transmitted to the air chuck 7 through the frictional member 9. Upon tightening the screw cap 12, the frictional member 9 is pressurized by a mechanical force such as an air force, a spring force or the like, and the screw cap is tightened by an inertia of the spindle 8 and the air chuck 7. At this time, when the tightening torque reaches a predetermined value, that is, when its reactive force exceeds a static frictional force between the frictional member 9 and the retainer plates 10, a slippage between the frictional member 9 and the retaining plates 10 will be generated to thereby prevent any tightening torque, above the predetermined value, from being generated. In this case, although the engagement between the frictional member 9 and the retainer plates 10 may be of the dry type, a lubricant oil is frequently injected between the frictional member 9 and the retainer plates 10 in order to avoid a melt-sticking damage. It is, therefore, very difficult to always keep the tightening torque constant due to a change in inertia moment caused by a change, in rotational speed, of the spindle 8, a change in frictional coefficient caused by a temperature change of the frictional member 9, and another change in frictional coefficient caused by a temperature change of the lubricant oil and in accordance with an amount and a kind of the lubricant oil. Thus, it is very difficult to adjust a pressure applied to the frictional member 9 every time the various factors are changed. Also, it is general that such a tightening apparatus is of the multi-head type provided with a plurality of spindles 8, and hence, it is very difficult to adjust the frictional coefficient for the respective frictional members 9.
As described above, the conventional tightening apparatus encounters the serious disadvantages, due to the various factors, that it is difficult to screw-tighten a component, to be screw-tightened, at a constant tightening torque, a tightening torque applied to a component to be closed is varied in a wide range, and it is insufficient to control the tightening torque.